[nfc][libomptarget] Reorganise support header

Summary:
[nfc][libomptarget] Reorganise support header

All functions defined in support implementation are now declared in support.h
Reordered functions in support implementation to match the sequence in support.h
Added include guards to support.h
Added #include interface to support.h to provide kmp_Ident declaration
Move supporti.h to support.cu and s/INLINE/EXTERN/g
Add remaining includes to support.cu

A minor side effect is to change the name mangling of the support functions to
extern "C". If this matters another macro along the lines of INLINE/EXTERN
can be added - perhaps DEVICE as that's the obvious implementation.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D69652